This invention was made with Government support. The Government has certain rights in this invention.
1. Field of the Invention
The invention relates generally to optical fiber communication systems and, more particularly, to alignment systems and methods therefor.
2. Description of the Related Art
Optical fiber communication systems are presently being designed for free-space transmissions over long (unrepeated) distances. These communication systems include multiple fiber-based communication terminals that transmit and receive very narrow optical signals (i.e., beams) to avoid reliance on the high-power beams utilized heretofore. To transmit and receive such narrow beams in free space accurately, optical communication terminals require extremely precise alignment systems. For instance, a typical fiber-based communication terminal is designed to utilize a telescope having a field of view on the order of several microradians. With the telescope providing such a small target, the alignment system must compensate for numerous factors, including, for example, relative movement of the receiving terminal subsequent to transmission. Such compensation for relative movement between the terminals requires the transmitting terminal to calculate a xe2x80x9cpoint-aheadxe2x80x9d angle for transmission.
Prior proposed free space optical communication systems have relied on open-loop control for calculation of the point-ahead angle, but thermal effects, launch environment effects, actuator variation, and/or aging may cause the beam to drift away from the desired point-ahead angle. Thermal effects, in particular, may easily cause the relative positioning of various components in the communication terminal to change dramatically. These effects not only frustrate long-term alignment calibration for signal transmission, but also complicate alignment calibration for reception of an incoming beam that has accurately hit the telescope. Thus, alignment calibration challenges persist over the lifetime of the communication system for both signal transmission and reception.
The field of potential alignment calibration solutions is generally restricted by the nature of the communication terminal, inasmuch as the alignment system typically resides in the communications payload of a space-borne vehicle (i.e., a satellite). Competition exists between the communications payload and other equipment on the satellite as a result of limitations on both the size and weight of the payload.
In accordance with one aspect of the present invention, an optical communication terminal useful for transmitting data via an optical signal includes a transmit mirror positioned to direct the optical signal, a processor configured to develop a control signal to establish an optimal position of the transmit mirror, and a mirror controller coupled to the transmit mirror and responsive to the control signal. The optical communication terminal further includes a modulator coupled to the processor that modulates the optical signal in accordance with a position of the transmit mirror during transmission of the optical signal.
In accordance with another aspect of the present invention, an alignment calibration system is useful in a first optical communication terminal communicating with a second optical communication terminal. The inventive system includes a modulator that modulates a first data signal with a first alignment signal representative of an alignment offset taken by the first optical communication terminal during transmission of the first data signal by the first optical communication terminal to the second optical communication terminal. The inventive system further includes a demodulator responsive to a second data signal transmitted by the second optical communication terminal. The demodulator develops a second alignment signal representative of the alignment offset taken by the first optical communication terminal and an intensity of the first data signal as received by the second optical communication terminal. The inventive system still further includes a processor coupled to the demodulator and configured to determine an optimal alignment position from the alignment offset and the intensity.
In accordance with yet another aspect of the present invention, a method is useful for aligning a first communication terminal with a second communication terminal for optical transmission of data between the first communication terminal and the,second communication terminal. The inventive method includes the steps of adjusting a mirror of the first communication terminal to a first position, modulating a first optical signal representative of the data in accordance with the first position, and transmitting the first optical signal from the first communication terminal to the second communication terminal. A second optical signal from the second communication terminal is then received to obtain an intensity of the first optical signal as received by the second communication terminal. The inventive method then includes the step of adjusting the mirror of the first communication terminal to a second position.
In accordance with still another aspect of the present invention, a method is useful for aligning a first communication terminal and a second communication terminal. The inventive method includes the steps of adjusting a mirror of the first communication terminal to a plurality of predetermined offset positions from a first position, and transmitting, for each predetermined offset position, a respective first optical signal from the first communication terminal to the second communication terminal wherein each first optical signal is representative of the corresponding predetermined offset position. The inventive method further includes the step of receiving, for each predetermined offset position, a respective second optical signal from the second communication terminal to the first communication terminal wherein each second optical signal is representative of a respective intensity of the corresponding first optical signal as received by the second communication terminal. The inventive method still further includes the step of adjusting the mirror of the first communication terminal to a second position based on the plurality of predetermined offset positions and the corresponding received intensities.
In accordance with yet a further aspect of the present invention, a method of calibrating an alignment system for an optical communication terminal transmitting content via a data signal at a carrier frequency includes the steps of generating a digital alignment signal representative of alignment information, and frequency-modulating a subcarrier signal having a frequency lower than the carrier frequency with the digital alignment signal. The inventive method further includes the steps of modulating the data signal with the frequency-modulated subcarrier signal and transmitting the modulated data signal.
In accordance with still a further aspect of the present invention, a method is useful for controlling a mirror positioning system for an optical communication terminal. The inventive method includes the step of receiving intensity data associated with optical transmission of content at a plurality of mirror offset positions established by the mirror positioning system. The method further includes the step of determining an optimal mirror position to maximize transmission intensity via a parabolic fit of the intensity data.